c++ standard, section 8.5.3/5:
A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows: ... blah ... blah ..
— Otherwise, the reference shall be to a non-volatile const type (i.e., cv1 shall be const). [Example...
This one cost me 32 hrs of digging in gdb, google, etc. and finally had to hit IRC to figure out. Shyte.